Why do I need my camera settings for Motion Tracking VFX
If you are just using 2D elements like planar tracking you don't need to deal with the camera settings.

If you are planing to reconstruct a scene by creating a virtual camera that mimics the camera used to record a video then you need to know the dimensions  camera sensor and have lens information so that blender can reconstruct the scene properly.

The combination of lens size and sensor size determines the angle of coverage of the camera and the perspective. Blender can use such information to then determine the placement of the tracked elements in a 3 dimensional space. If you don't know such information blender can guess it so some degree, but you might get better camera solutions if you start by supplying such info.

Camera solving is the process described above. From the movement of trackers on a a series of flat images, blender estimates the placement of the tracked points in  3D space based the distance to the camera and also makes the virtual camera move like the original camera used to make the shot.

I recommend that you watch a few videos of how it is done to familiarize with the process. 

Please read the following post for tips on how to get better camera solutions:

How can I get better results when doing camera motion tracking?